Up to now, the economic treatment of gas mixtures, such as occur for example in the petroleum industry, but also in petrochemicals, represents a problem which has not yet been satisfactorily solved. Processes using the membrane separating technique for the separation or recovery of gases of high purity, greater than 99%, are fundamentally known, particularly in the combination of membrane separating devices with pressure swing adsorption devices. Where high product purities are demanded, exclusively membrane separating processes are classified as being uneconomic because the multi-stage recirculation of the permeate streams demand an excessively high expenditure of energy and excessively high costs for recompression. The combination of membrane separating devices and pressure swing adsorption devices can only be economically operated where the initial content of desired gases to be separated from the gas/vapour mixture, for example H2, is above 40 mole % and where a high feed pressure is available. Because the glassy polymer membranes previously employed for this purpose in the membrane separating device must, as a precondition, exhibit selectivity which is as high as possible despite low fluxes, it is necessary to set the highest possible pressure to the inlet of the separating device, for example between 40 and 130 bar, in order to be able to utilise the selectivity of the membranes. The downstream pressure swing adsorption device was likewise operated in the known arrangements at a relatively high adsorption pressure, preferably 40 bar and higher. The compression costs are very high so that the total separating process can only be operated economically when the mixture to be separated is already available at high pressure.
Processes of the type previously described are known, for example, from U.S. Pat. No. 4,398,926 and U.S. Pat. No. 4,690,695. In EP-A-0684066, the recovery of H2 or He is known from high pressure supply flow in which the inlet concentration of H2 or He can be below 30% by volume, where the impurities are caused by higher hydrocarbons. Under these conditions, the hydrogen can be raised in a membrane unit to 40% in the permeate before the permeate is supplied, after recompression, to a pressure swing adsorption device in which the light component, for example H2, is enriched to over 98%. The energy consumption for the recompression is controlled, at the expense of the yield, by extracting only a small permeate flow.
It is, therefore, an object of the present invention to create a process by means of which, at low energy requirements, simultaneous separation or recovery of high purity gases having a small kinetic diameter and of condensable gases/vapours in highly concentrated form can be achieved by applying moderate pressure only, it being possible for the gas and/or gas vapour mixture which has to be separated to have a low concentration of the components to be enriched:
The object is achieved, according to the invention, by at least one membrane separating unit having an organophilic membrane which is selective for C2 and higher hydrocarbons combined with at least one membrane separating unit having a glassy membrane which is selective for gases of small kinetic diameter to which is supplied at the inlet end the retentate from the organophilic membrane separating unit.
The advantage of the process according to the invention lies essentially in the fact that, as desired, simultaneous economical separation and/or recovery of gases with a small kinetic diameter such as hydrogen or helium and condensable gases/vapours, such as C2 and higher hydrocarbons from gas and/or gas vapour mixtures is possible. From the point of view of ecology also, the process according to the invention is advantageously characterised by substantially reduced energy requirements as compared with known processes because the process can be operated at relatively moderate pressures. The process is also advantageously applicable where the gases/vapours to be separated are only available in small or even in only very small molar concentrations in the mixture.